This proposal is concerned with the application of paramagnetic relaxation studies to the active centers of low spin hemoproteins. Only two such measurements (one by the author of this proposal) have ever been reported and those data indicate that: (1) the observed temperature dependence is anomalous, and (2), there are substantial differences in the relaxation of the two systems which are apparently associated with the ligand structure about the low spin ferric ion. We propose to measure the paramagnetic relaxation rate of low spin Fe 3 ion in 8 hemoproteins or relate model compounds spanning the five classes designated by Blumberg and Peisach on the basis of their axial ligands and associated g-factors. These data will then provide some understanding of the coupling between the vibrational spectrum of the structure and relaxation of the ferric magnetic moment. In addition these data will determine if the relaxation rate of the iron, which is a measure of the dynamic ligand field at the iron site, is a more sensitive probe of the axial ligand structure, than the principal g-factors, which measure the time averaged ligand field. The most appropriate phenomenological expression for the Raman (two phonon) relaxation rate will be determined and the parameter sets which best fit the relaxation data of the 8 compounds will then be correlated with the known structure of the hemoprotein. In addition to establishing the applicability of paramagnetic relaxation as a structural probe of hemoproteins, the data will be useful to investigators using NMR, EPR, ENDOR, and Mossbauer spectroscopy. The relaxation rates will be measured using the pulse saturation/recovery technique at controlled temperatures below 25 K and a microwave frequency near 9.5 GHz.